Documentation: Fix index.rst after psci.rst renaming

Fix the TOC in index.rst after psci.rst has been renamed to
hypercalls.rst.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: https://lore.kernel.org/r/20220504205627.18f46380@canb.auug.org.au